Production of soft cold reduced steel



Patented June 12, 1945 PRODUCTION OF SOFT COLD REDUCED STEEL Frederick G. Campbell and Edward L. Robinson, Apollo, Pa.

No Drawing. Application May 5, 1941, Serial No. 391,984

6 Claims. (01. 148--12) This invention relates to the production of flat ferrous material, such as strips and sheets, which, while being produced in accordance with approved cold mill practices for reasons of efficiency and economy, is possessed of the properties of softness and formability usually associated with similar material produced by hot mill methods.

- The general sheet metal trade requires a galvanizedsteel sheet for fabrication into a wide variety of metal shapes and structures. The formation of the material into such articles is usually accomplished by hand tools for bending, etc., and, consequently, the property primarily required in material for this application is that it possess softness. This term is applied in th popular sense to indicate a plasticity, a lack of stiffness, and freedom from spring back (tendency to return to pre-formed position) after being bent from its original contour (distinct from metallurgical qualities of hardness value, ductility elonration)- Steel sheet material of this kind heretofore supplied to the trade, which is intended subsequently to receive a protective galvanized coating, has long been produced as the sheet product of the hand operated hot mills. More recently this material has been rolled in continuous hot strip mills in order to obtain the economic advantages afforded by this practice.

it. has been found, however, that the hot rolled material, whether produced as sheets or as continuous strip, lacked some degree of uniformity in the desired physical and metallurgical properties.

C'old reduction methods, being known to provide uniformity in the physical properties'of fiat material produced thereby, have been contemplated for the production of this class of product,

but, even when followed by one or more annealing operations, the resulting cold reduced material has not manifested the requisite softness nor metallurgical properties. The economics and efficiencies derivable from such cold reduction methods have, therefore, not been available to the producers of this class of'product in the past.

We have discovered that by selecting a steel according to certain considerations hereinafter described and subjecting this steel to certain hot rolling, cold reduction and heat treatment, the requisite characteristics can be imparted to the material, and metallurgical and physical qualities uniformly maintained. Thus, we are enabled to produce steels of this kind which not only possess improved physical charactersistics, but which may be produced by cold-reduction methods with their attendant economies, as well.

It is, therefore, the principal object of this invention to provid a novel method and product which combine the benefits of cold reduction methods with hot mill results, ancillary objects and advantages being implicit in the ensuing disclosure,

The physical properties sought in steels of this type, more commonly referred to as "jobbers stock, are usually associated with the metallugrapliic condition in the metal known as large grains, a characteristic common to hot-rolled steels that ar finished above the temperature of recrystallization. Generally speaking, the finer the grain, the harder and stiller the material.

Due to limitations that inhere in heat treatin practices of cold reduced sheets necessary for the attainment of completely annealed (stress-free) material, i. e., the normalizing or annealing of cold reduced material as today known and practiced, steels of relatively small grain size, medium softness, and ductility, though still having appreciable degree of stifiness, are produced. We, on the other hand, have discovered ways of utilizing the advantages of cold reduction and heat treatment without incurring the fine grain conditions usually associated with steels produced in this manner. Sheets produced in accordance with our invention are of such large grain size as to have, in addition to sumcicnt ductility, practically maximum softness and lack of springiness desired in products of this kind. Our material can be formed with the facility and with the permanence of deformation (no spring-back) that have heretofore characterized the hot-rolled material produced for like purposes.

of the latter Though we have found that the silicon content can be increased up to 0.20 per cent without substantially afiecting the results produced, we prefer to keep the silicon at a minimum.

In rimmed steel ingots there always are present certain irregularities in the structure and in the a chemical composition. These irregularities carry through the rolling and heat treatment of the sheet product, the rimmed portion containing substantially less carbon, phosphorus, and sulphur, as well as oxygen, than the inner portion or core of the metal. In addition to this segregated condition of the constituents,- the core has a different grain structure. Our first considerartion', accordingly, is to select those ingots which have a minimum of segregation, inclusions and mechanical defects and to discard, for our purposes, the top portion amounting to about onethird of the whole.

a The next step in our processing is ,the continu-v ous hot rolling of the ingot down to a thickness several times that of the desired finished sheet gauge which latter may range from 14 gauge (0.080 inch) to 30 gauge (0.0140 inch). In the composition is rejected by each individual grain of austenite to the grain boundaries and the carbon collects in areas having eutectoid composition. If the steel, now in the form of strip, is subjected to further hot rolling the result is, in effect,

similar to that produced by cold working. The

lower the finishing temperature in the hot mill, the greater is the tendency for banding of the segregates and inclusions and for grainsize contrast. This is particularly true in'the case of rimmed steel.

Such stringers or bands remain in the hot strip material during the subsequent cold reduction and during the critical strain and heat treatment. The presence of these stringers tends to arrest or retard the progress of grain growth, which originates at the surface of the sheet, the propagation of which is prevented from continuing, through the cross section. This is particularly the case when rimmed steel is treated, due to the irregularities in composition and structure between the rimmed and the central portions of the metal as it has solidified.

In our processing, the condition is prevented from developing by finishing the hot rolling operation at a temperature above that at which the structure remains austenitic. The hot rolled strip is then coiled at a temperature of not less than 1225 F.1275 F. Upon further cooling, the carbides are retained in the form of fine well dispersed particles. In this form, it is found that the subsequent cold rolling does not cause the grain growth-retarding stringers or bands to develop.

The hot rolled strip is then cold reduced, on

the order of to in order that a later 45 temper pass rolling, ranging from 2% to 10% elongation, will bring the strip material to the desired finished gauge. In the heavy cold reduc-v tion employed," the original grains are completely fragmented and. the carbides remain finely dispersed.

The heavy cold reduction not only provides a means of control by which good surface and uniformity of gauge are obtained, but also facilitates the development of uniform grain size through.- 9

out the cross section of the material in response to the annealing treatment which follows.

The annealing treatment involves heating the material and maintaining it at a temperature below the upper critical point, preferably between 1200 F. to 1300" F., for a period of time varying with the type of furnace. Single thicknesses of the material may be annealed continuously (hence rapidly), while box annealing will require not less than twelve hours. The material is then permitted to cool normally, at a rate determined by the mass of material as used in commercial annealing practices. It will be understood that the heating cycle for killed steels will be somewhat longer than thatfor rimmed steels, due to the inherently sluggish manner in which the fine grains of such steels recrystallize.

This treatment is followed by a temper rolling, chosen so as to reduce the material to the desired finished gauge and at the same time to impart a critical strain to the metal. Preferably this rolling operation will elongate the material a amount ,ranging from 2% to 10%.

'The critically strained material is then reheated, slowly, to a temperature below the upper critical point, and within the range of germinative grain growth temperature corresponding to the degree of critical strain developed. The upper temperature point of the range is limited only by the tendency of adjacent surfaces, to form stickers? The lower temperature is limited by the practical cycle below which the crystallization rate becomes sluggish.

Usually, for steels of the analysis given, which have been critically strained to about 7% to 8% extension, this-temperature will be around 1250 F. It will be understood that the less the amount of critical strain imparted to the metal, the higher the critical annealing temperatures, or the longer the soaking period at the same temperatures, should be. The normal holding time at temperature may vary from 8 to 14 hours, dependent upon the strain and temperature factors, mentioned above.

The application of this treatment to low carbon rimmed or killed steel results in the formation of uniform exceedingly coarse grains in the metal with the accompanying coarse grains in the metal of low yield point, and low spring back" upon being subjected to forming operations.

Comparative tests for physical properties and structure on treated and upon untreated material would conform to the following:

Untreated'materi'al cold reduced, box annealed, temper rolled,

355 gal. elongated few 7.

Material treated according to this invention:

Ult. Elong. Rockint, strength, in 2 well Olsen Microstructuro I sq. in. #/sq. in. per cent 8" l. 16, 38, 500 34. 0 22.0 300 3-4. 2. 2, 9U 36, 200 39. 0 l4. 0 ,330 l-2. 3. P 14, 213 35,900 36. 0 l8. 0 334 2-3 max. 6. 4- 14, 300 36, 400 35. 6 l8. 0 338 l-2 vfew 5. 5 i2, 500 35. 100 33. 0 l7. 0 358 l-2. 6. 16, 300- 38, 600 38. 0 l2. 6 342 2-3 low 5.

' ards of the American Society for Testing Materials. Thus, number 6 and 7 grains means approximately 32 to 64 grains per square inch of viewing surface magnified 100 diameters. Grain size desi nated as number 1 or 2 means one or two grains per square inch, at a magnification of the surface viewed of 100 diameters. From these tables, the large grain, soft characteristics of steels produced in accordance with this invention modifications of the invention may be made without departing from the scope thereof, and it is not intended that we belimited to the specific embodiment shown, other than as is rendered necessary by the recitations of the appended claims:

We claim:

1. The method of producing by cold reduction methods, plain low carbon soft steel flats containing substantially 0.06 to 0.10 per cent carbon and not more than 0.2 per cent silicon, which flats are free from tendencies to recover preformed position upon bending, which method comprises selecting a mild steel cast body relatively free from segregation and inclusions, hotworking the said body to intermediate gauge to finish above the transformation temperature, cold-working the said body to within temper-pass .of final'gauge, annealing the said body below the upper critical temperature, cold-working the said body to gauge so as critically to strain the metal, and annealing the critically strained body at temperatures provident of germinative grain growth ,2. The method of producing by cold reduction methods, plain low carbon soft steel flats containing substantially 0.06 to 0.10 per cent carbon,

and not more than 0.2 per cent silicon, which flats are free from tendencies to recover preformed position upon bending, which method includes selecting a mild steel cast body relatively free-from segregation and inclusions, hot-working the said body' to intermediate gauge to finish working the said body to cause fragmentation of the rains thereof, annealing the said body at a temperature and for a period of time at which recrystallization takes place, imparting a critical strain to the metal by cold-working up to 10 per cent extension, and annealing the body at a temperature above approximately 1200" 1". for from 8 to 1'4 hours, so that the time-temperature factor varies inversely as the amount of strain in the metal at the beginning of this annealing operation.

3. In the artof producing plain low carbon soft steel sheets and strip containing substantially 0.06 to 0.10 per cent carbon and not more than 0.2 per cent silicon and which are free from tendencies to recover preformed position upon bend ing, the improvement which comprises hot-working the said body to intermediate gauge to finish above the austenitic transformation temperature,

. cold-workingthe said body substantially to gauge,

annealing the said body below the upper critical temperature, cold-reducing the said body .to gauge so as critically to strain the metal, and annealing the critically strained body at temperatures provident of germinative grain growth.

4, In the art of producing plain low carbon soft steel sheets and strip containing substantially 0.08 to 0.10 per cent carbon and not more than above the transformation temperature, cold- 0.2 per cent silicon, which sheets and strip are free from tendencies to recover preformed position upon bending, the improvements which co'mprise hot-working the said body to intermediate gauge to finish above the austenitic transformation temperature, cold-working the said body to cause fragmentation of the grains thereof, anhealing the said body below the upper critical temperature, imparting a critical strain to the metal by cold-working up to 10 per cent exten-- slon, and annealing the critically strained body at temperatures provident of germinative grain rowth.

5. The method of producing by cold-reduction methods, plain soft low'carbon steel flats containing substantially 0.06 to 0.10 percent carbon and not more than 0.2 per cent silicon, which flats are free from tendencies to recover preformed position upon bending, which method comprises selecting a mild steel cast body relatively free from segregations and inclusions, hot-working the body to intermediate gauge while maintaining the steel continuously during working above the austenitic transformation temperature, thereby retaining in solution any segregates and incluslons which may be present while also precluding banding and stringing, then, as deformation ceases, coiling at a temperature not less than crystallizationtakes place, imparting a critical strain to the metal by cold-working to from approximately 2 per cent to approximately 10 per cent elongation, and annealing the resulting critically strained body by heating the metal and. maintaining it at a temperature of approximately 1250 1". for from approximately 8 to 14 hours so that the time-temperature factor varies inversely as the amount of strain in the metal at the commencement of this annealing-operation.

6. As a new article'of manufacture, a fiat plain soft carbon steel body containing substantially 0.06 to 0.10 per cent carbon and not more than 0.2 per cent silicon. which body has been reduced to gauge from hot strip size by cold rolling, characterized by a dead softness, a microstructure composed of large uniform grains, and a freedom from grain growth inhibiting formations of segregates and'inclusions, which body is free from "springback upon being bent, and which has been produced by the method of claim 1. 

